Channel state information transmission/reception method and apparatus for use in wireless communication system

ABSTRACT

A method and an apparatus for transmitting/receiving channel state information for use in multi-antenna system are provided. A signal communication method of a base station having a plurality of antennas in a wireless communication system includes determining antenna ports of first and second directions based on directions of the plurality of antennas, allocating channel measurement resources for the respective antenna ports to a terminal, transmitting a feedback configuration to the terminal according to the channel measurement resources, and receiving feedback information from the terminal based on the channel measurement resource and the feedback configuration. The signal transmission/reception method and apparatus are advantageous in transmitting/receiving channel state information efficiently in the system using a plurality of antennas.

PRIORITY

This application is a continuation application of a prior applicationSer. No. 15/137,779, filed on Apr. 25, 2016, which has issued as U.S.Pat. No. 9,735,936 on Aug. 15, 2017, and which is a continuationapplication of prior application Ser. No. 13/915,044, filed on Jun. 11,2013, which issued as U.S. Pat. No. 9,326,176 on Apr. 26, 2016, andwhich claims the benefit under 35 U.S.C. §119(e) of a U.S. provisionalpatent application filed on Jun. 11, 2012 in the U.S. Patent andTrademark Office and assigned Ser. No. 61/658,033, the entire disclosureof each of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method and an apparatus for channelstate information transmission/reception. More particularly, the presentinvention relates to a method and an apparatus fortransmitting/receiving channel state information in multi-antennasystem.

2. Description of the Related Art

A Reference Signal (RS) is a signal used for measuring the channel state(or quality) between the base station and users by taking notice of asignal strength or distortion, an interference strength, and Gaussiannoise to help demodulating and decoding the data symbol at the receiver.Another use of the reference signal is to measure the radio channelstate. The receiver measures the received signal strength of thereference signal transmitted at the negotiated transmission power levelon the radio channel. The radio channel state determined in this way isused in order for the receiver to determine the data rate to berequested to the transmitter.

The next generation wireless mobile communication system standards, suchas the 3rd Generation Partnership Project (3GPP), the Long TermEvolution Advanced (LTE-A) and the Institute of Electrical andElectronics Engineers (IEEE) 802.16m adopt a multicarrier-based multipleaccess technique, such as Orthogonal Frequency DivisionMultiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA).In the case of the wireless mobile communication system adoptingmulticarrier-based multiple access, the channel estimation andmeasurement performance varies depending on the positions and number ofsymbols and subcarriers in time and frequency domains. Furthermore, thechannel estimation and measurement performance are influenced by thepower level allocated for the reference signal. Accordingly, as theresource amount (i.e., time, frequency, and transmission power)allocated for the reference signal increases, the channel estimation andmeasurement performance are improved, resulting in improvement ofdemodulation and decoding performance of the received data symbol andchannel state measurement accuracy.

In the typical mobile communication system operating with a limitedradio resource (i.e., time, frequency, and transmission power), however,if a large amount of radio resource is allocated for reference signaltransmission, the radio resource amount for data transmission decreases.For this reason, the radio resource allocation for reference signaltransmission has to be determined in consideration of the systemthroughput. More particularly, in a Multiple Input Multiple Output(MIMO) system for multi-antenna transmission, the resource allocationfor reference signal and measurement of resource signal are veryimportant technical factors.

Therefore, a need exists for a method and an apparatus for transmittingchannel state information efficiently.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present invention.

SUMMARY OF THE INVENTION

Aspects of the present invention are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentinvention is to provide a method and an apparatus for transmittingchannel state information efficiently.

Another aspect of the present invention is to provide a method and anapparatus for transmitting/receiving channel state informationefficiently in a system using a plurality of antennas.

In accordance with an aspect of the present invention, a signalcommunication method of a base station having a plurality of antennas ina wireless communication system is provided. The method includesdetermining antenna ports of first and second directions based ondirections of the plurality of antennas, allocating channel measurementresources for the respective antenna ports to a terminal, transmitting afeedback configuration to the terminal according to the channelmeasurement resources, and receiving feedback information from theterminal based on the channel measurement resource and the feedbackconfiguration.

In accordance with another aspect of the present invention, a signalcommunication method of a terminal in a wireless communication system isprovided. The method includes receiving channel measurement resourcesallocated for antenna ports formed in first and second directionsdetermined based on directions of a plurality of antennas, receiving afeedback configuration according to the channel measurement resources,and transmitting feedback information measured based on the channelmeasurement resource and the feedback information.

In accordance with another aspect of the present invention, a basestation of a wireless communication system is provided. The base stationincludes a transceiver having a plurality of antennas, and a controlunit, for controlling the transceiver, for determining antenna ports offirst and second directions based on directions of the plurality ofantennas, for allocating channel measurement resources for therespective antenna ports to a terminal, for transmitting a feedbackconfiguration to the terminal according to the channel measurementresources, and for receiving feedback information from the terminalbased on the channel measurement resource and the feedbackconfiguration.

In accordance with another aspect of the present invention, a terminalof a wireless communication system is provided. The terminal includes atransceiver for transmitting and receiving to and from a base stationhaving a plurality of antennas, and a control unit, for controlling thetransceiver, for receiving channel measurement resources allocated forantenna ports formed in first and second directions determined based ondirections of a plurality of antennas and a feedback configurationaccording to the channel measurement resources, and for transmittingfeedback information measured based on the channel measurement resourceand the feedback information.

The present invention is advantageous in transmitting/receiving channelstate information efficiently in the system using a plurality ofantennas.

Other aspects, advantages, and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainexemplary embodiments of the present invention will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a diagram illustrating a Full Dimension-Multiple InputMultiple Output (FD-MIMO) system according to an exemplary embodiment ofthe present invention;

FIG. 2 is a time-frequency grid illustrating a single resource block ofa downlink subframe as a smallest scheduling unit in a Long TermEvolution (LTE/LTE-Advanced (LTE-A) system according to an exemplaryembodiment of the present invention;

FIG. 3 is a diagram illustrating a mechanism of a Channel StatusInformation Reference Signal (CSI-RS) transmission in an FD-MIMO systemaccording to an exemplary embodiment of the present invention;

FIG. 4 is a diagram illustrating a procedure of estimating channelsbased on a Horizontal-CSI-RS (H-CSI-RS) and a Vertical-CSI-RS (V-CSI-RS)and generating channel state information according to an exemplaryembodiment of the present invention;

FIG. 5 is a timing diagram illustrating a channel state informationfeedback method 1 according to an exemplary embodiment of the presentinvention;

FIG. 6 is a timing diagram illustrating a channel state informationfeedback method 2 according to an exemplary embodiment of the presentinvention;

FIG. 7 is a timing diagram illustrating a channel state informationfeedback method 3 according to an exemplary embodiment of the presentinvention;

FIG. 8 is a timing diagram illustrating a channel state informationfeedback method 4 according to an exemplary embodiment of the presentinvention;

FIG. 9 is a block diagram illustrating a configuration of an evolvedNode B (eNB) according to an exemplary embodiment of the presentinvention;

FIG. 10 is a block diagram illustrating a configuration of a UserEquipment (UE) according to an exemplary embodiment of the presentinvention;

FIG. 11 is a flowchart illustrating an eNB procedure according to anexemplary embodiment of the present invention; and

FIG. 12 is a flowchart illustrating a UE procedure according to anexemplary embodiment of the present invention.

Throughout the drawings, it should be noted that like reference numbersare used to depict the same or similar elements, features, andstructures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of theinvention. In addition, descriptions of well-known functions andconstructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of theinvention. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent invention is provided for illustration purpose only and not forthe purpose of limiting the invention as defined by the appended claimsand their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

By the term “substantially” it is meant that the recited characteristic,parameter, or value need not be achieved exactly, but that deviations orvariations, including for example, tolerances, measurement error,measurement accuracy limitations and other factors known to those ofskill in the art, may occur in amounts that do not preclude the effectthe characteristic was intended to provide.

Although the following description is directed to an OrthogonalFrequency Division Multiplexing (OFDM)-based radio communication system,particularly the 3rd Generation Partnership Project (3GPP) and anEvolved Universal Terrestrial Radio Access (EUTRA), it will beunderstood by those skilled in the art that exemplary embodiments of thepresent invention can be applied even to other communication systemshaving similar technical background and channel format, with a slightmodification, without departing from the spirit and scope of the presentinvention.

Exemplary embodiments of the present invention relate to a method fortransmitting channel state information from the base station to theterminal for use in channel quality measurement in the radio mobilecommunication system adopting a multi-carrier multiple access scheme,such as Orthogonal Frequency Division Multiple Access (OFDMA).

The mobile communication system has evolved into a high-speed,high-quality wireless packet data communication system to provide dataand multimedia services beyond the early voice-oriented services. Inorder to fulfil the requirements for such services, the standardorganizations, such as 3GPP, 3GPP2, and the Institute of Electrical andElectronics Engineers (IEEE), are in the middle of standardization ofmulticarrier-based multiple access next generation mobile communicationsystem. Recently, various mobile communication standards, such as LongTerm Evolution (LTE) of 3GPP, Ultra Mobile Broadband (UMB) of 3GPP2, and802.16m of IEEE, have been developed to support the high-speed,high-quality wireless packet data communication services.

The 3G-evolved mobile communication systems of the related art, such asLTE, UMB, and 802.16m, are designed based on the multicarrier multipleaccess scheme. In addition, the 3G evolved mobile communication systemsadopt various technologies including Multiple Input Multiple Output(MIMO), beamforming, Adaptive Modulation and Coding (AMC),channel-Sensitive scheduling, etc. Such technologies are capable ofconcentrating a transmission power to certain antennas or adjusting thetransmission data amount so as to increase the data transmission amountfor good channel condition, resulting in improvement of systemthroughput. Since most of these techniques operate based on the channelstate information between a Base Station (BS) or an evolved Node B (eNB)and a terminal (i.e., a User Equipment (UE) or a Mobile Station (MS)),the base station or the terminal has to measure the channel state and,at this time, the Channel Status Information Reference Signal (CSI-RS)is used for channel state measurement. The aforementioned base stationdenotes a device for transmitting in downlink and receiving in uplink,and a base station manages multiple cells for transmission/reception.The mobile communication system includes a plurality of base stationsdistributed geometrically, and each base station performstransmission/reception in a plurality of cells.

The 3G and 4G mobile communication systems of the related art, such asan LTE/LTE-Advanced (LTE-A) system, adopt MIMO technology to improve thedata rate and system throughput. The MIMO technique includestransmitting a plurality of information streams separated spatiallyusing multiple transmit/receive antennas. This technique of transmittingspatially separated multiple information streams is referred to asspatial multiplexing. Typically, the number of information streams forspatial multiplexing is determined depending on the numbers of transmitand receive antennas. Typically, the number of information streams thatcan be spatially multiplexed is referred to as a rank of thecorresponding transmission. In the MIMO technique supported by theLTE/LTE-A Release 11, the spatially multiplexing is up to 8transmit/receive antennas, and this means that the transmission rank issupported up to 8.

The massive MIMO or Full Dimension MIMO (FD-MIMO) system adopting thetechnology proposed in exemplary embodiments of the present inventionsupports 32 or more transmit antennas as compared to the legacyLTE/LTE-A MIMO technique supporting up to 8 transmit antennas. However,exemplary embodiments of the present invention are not limited thereto.

FIG. 1 is a diagram illustrating an FD-MIMO system according to anexemplary embodiment of the present invention. The FD-MIMO system is aradio communication system for transmitting data using a few dozen ormore transmit antennas.

Referring to FIG. 1, a base station transmitter 100 transmits radiosignals 120 and 130 through a few dozen or more transmit antennas.Transmit antennas 110 are arranged at a minimum distance among eachother. The minimum distance may be half of the wavelength (λ/2).Typically, in the case that the transmit antennas are arranged at thedistance of half of the wavelength of the radio signal, the signalstransmitted by the respective transmit antennas are influenced by radiochannel with low correlation. Assuming a radio signal band of 2 GHz,this distance is 7.5 cm and shortened as the band becomes higher than 2GHz.

In FIG. 1, a few dozen or more transmit antennas 110 arranged at thebase station are used to transmit signals 120 and 130 to one or moreterminals. In order to transmit signals to a plurality of terminalssimultaneously, an appropriated precoding is applied. At this time, oneterminal may receive a plurality of information streams. Typically, anumber of information streams which a terminal can receive is determineddepending on the number of receive antennas of the terminal, a channelstate, and a reception capability of the terminal.

In order to implement the FD-MIMO system efficiently, the terminal hasto measure the channel condition and interference size accurately andtransmit the channel state information to the base station efficiently.If the channel state information is received, the base stationdetermines the terminals for downlink transmission, downlink data rate,and precoding to be applied. In the case of FD-MIMO system using a largenumber of transmit antennas, if the channel state informationtransmission method of the legacy LTE/LTE-A system is applied withoutmodification, the control information amount to be transmitted in uplinkincreases significantly, resulting in uplink overhead.

In order to transmit information from the transmitter to the receiver inthe mobile communication system, a resource, such as time, frequency,and transmission power is used. The frequency and power are limitedresources. Accordingly, if the resource allocated for a reference signalincreases, the resource amount to be allocated for data traffic channeltransmission decreases, resulting in reduction of data transmissionamount. In this case, although the channel estimation and measurementperformance are improved, the data transmission amount decreases,resulting in reduction of an entire system throughput. Thus, it isnecessary to adjust the resource allocation amount for a referencesignal and a traffic channel transmission appropriately.

FIG. 2 is a time-frequency grid illustrating a single resource block ofa downlink subframe as a smallest scheduling unit in a LTE/LTE-A systemaccording to an exemplary embodiment of the present invention. In theLTE system, a subframe consists of a plurality of resource blocks.

Referring to FIG. 2, one of a plurality of resource blocks included in asubframe is illustrated. The radio resource is of one subframe in thetime domain and one RB in the frequency domain. The radio resourceconsists of 12 subcarriers in the frequency domain and 14 OFDM symbolsin the time domain, i.e., 168 unique frequency-time positions. In theLTE/LTE-A system, each frequency-time position is referred to as aResource Element (RE).

The radio resource structured as shown in FIG. 2 can be used fortransmitting a plurality of different types of signals as follows.

1. A Cell-specific Reference Signal (CRS): a reference signaltransmitted to all the UEs within a cell.

2. A Demodulation Reference Signal (DMRS): a reference signaltransmitted to a specific UE.

3. A Physical Downlink Shared Channel (PDSCH): a data channeltransmitted in downlink which the eNB use to transmit data to the UE andmapped to REs not used for reference signal transmission in data regionof FIG. 2.

4. A Channel Status Information Reference Signal (CSI-RS): a referencesignal transmitted to the UEs within a cell and used for channel statemeasurement. Multiple CSI-RSs can be transmitted within a cell.

5. Other control channels (i.e., a Physical Hybrid Automatic RepeatreQuest (HARQ) Indicator Channel (PHICH), a Physical Control FormatIndicator Channel (PCFICH), a Physical Control Channel (PDCCH), and thelike): channels for providing a control channel used for the UE toreceive PDCCH and transmitting an ACKnowledgement/Non-ACKnowledgement(ACK/NACK) of a HARQ operation for uplink data transmission.

In addition to the above signals, zero power CSI-RS can be configured inorder for the UEs within the corresponding cells to receive the CSI-RSstransmitted by different eNBs in the LTE-A system. The zero power CSI-RS(muting) can be mapped to the positions designated for CSI-RS, and theUE receives the traffic signal skipping the corresponding radio resourcein general. In the LTE-A system, the zero power CSI-RS is referred to asmuting. The zero power CSI-RS (muting) by nature is mapped to the CSI-RSposition without transmission power allocation.

In FIG. 2, the CSI-RS can be transmitted at some of the positions markedby A, B, C, D, E, F, G, H, I, and J according to the number of number ofantennas transmitting CSI-RS. In addition, the zero power CSI-RS(muting) can be mapped to some of the positions A, B, C, D, E, F, G, H,I, and J. The CSI-RS can be mapped to 2, 4, or 8 REs according to thenumber of the antenna ports for transmission.

For two antenna ports, half of a specific pattern is used for CSI-RStransmission, for four antenna ports, all of the specific pattern isused for CSI-RS transmission, and for eight antenna ports, two patternsare used for CSI-RS transmission. Meanwhile, muting is always performedby pattern. For example, although the muting may be applied to aplurality of patterns, if the muting positions mismatch CSI-RSpositions, it cannot be applied to one pattern partially.

In the case of transmitting CSI-RSs of two antenna ports, the CSI-RSsare mapped to two consecutive REs in the time domain and distinguishedfrom each other using orthogonal codes. In the case of transmittingCSI-RSs of four antenna ports, the CSI-RSs are mapped in the same way ofmapping the two more CSI-RSs to two more consecutive REs. This isapplied to the case of transmitting CSI-RSs of eight antenna ports.

In a cellular system, the reference signal has to be transmitted fordownlink channel state measurement. In the case of the 3GPP LTE-Asystem, the UE measures the channel state with the eNB using the CSI-RStransmitted by the eNB.

The channel state is measured in consideration of a few factorsincluding downlink interference. The downlink interference includes theinterference caused by the antennas of neighbor eNBs and thermal noisethat are important in determining the downlink channel condition. Forexample, in the case that the eNB with one transmit antenna transmitsthe reference signal to the UE with one receive antenna, the UE has todetermine energy per symbol that can be received in downlink andinterference amount that may be received for the duration of receivingthe corresponding symbol to calculate Es/Io from the received referencesignal. The calculated Es/Io is reported to the eNB such that the eNBdetermines the downlink data rate for the UE.

In the LTE-A system, the UE feeds back the information on the downlinkchannel state for use in downlink scheduling of the eNB. For example,the UE measures the reference signal transmitted by the eNB in downlinkand feeds back the information estimated from the reference signal tothe eNB in the format defined in the LTE/LTE-A standard. In theLTE/LTE-A system, the UE feedback information includes the followingthree indicators:

1. A Rank Indicator (RI): a number of spatial layers that can besupported by the current channel experienced at the UE.

2. A Precoding Matrix Indicator (PMI): a precoding matrix recommended bythe current channel experienced at the UE.

3. A Channel Quality Indicator (CQI): a maximum possible data rate thatthe UE can receive signal in the current channel state. A CQI may bereplaced with the Signal Interference to Noise Ratio (SINR), a maximumerror correction code rate and modulation scheme, or per-frequency dataefficiency that can be used in similar way to the maximum data rate.

The RI, PMI, and CQI are associated among each other in meaning. Forexample, the precoding matrix supported in LTE/LTE-A is configureddifferently per rank. Accordingly, the PMI value ‘X’ is interpreteddifferently for the cases of RI set to 1 and RI set to 2. In addition,when determining CQI, the UE assumes that the PMI and RI which it hasreported is applied by the eNB. For example, if the UE reports RI_X,PMI_Y, and CQI_Z, the UE is capable of receiving the signal at the datarate corresponding to CQI_Z when the rank RI_X and the precoding matrixPMI_Y are applied. In this way, the UE calculates CQI with which theoptimal performance is achieved in real transmission under theassumption of the transmission mode to be selected by the eNB.

In LTE/LTE-A, a CSI Process is defined as a means for configuring thechannel state information, such as CQI, RI, and PMI to the UE. The CSIprocess includes a CSI-RS and an Interference Measurement Resource(IMR). The eNB is capable of configuring at least one CSI-RS per UE, andthe UE measures the CSI-RS designated in the CSI process to calculatethe received signal strength passed the channel and measures the IMR tocalculate the strength of interference influencing the received signal.The IMR is the radio resource configured separately for interferencemeasurement of the UE such that the UE assumes all signals received onthe IMR are interference. In addition, an IMR matches one of A to H towhich muting (zero power CSI-RS) may be applied. For example, if the eNBconfigure the radio resource of B as IMR in FIG. 2, the UE performsinterference measurement on the radio resource of B in every RB.

Typically, in the FD-MIMO using a plurality of transmit antennas, thenumber of CSI-RSs has to increases in proportion to the number oftransmit antennas. In an exemplary case of LTE/LTE-A using 8 transmitantennas, the eNB has to transmit CSI-RSs of 8 ports to the UE fordownlink channel state measurement. At this time, in order to transmit8-port CSI-RSs, 8 REs has to be allocated for CSI-RS transmission in oneRB. For example, the REs indicated by alphabets A and B can be used forCSI-RS transmission of the corresponding eNB. In the case of applyingCSI-RS transmission scheme of LTE/LTE-A to FD-MIMO, the CSI-RStransmission resource increases in proportion to the number of transmitantenna. For example, the eNB having 128 transmit antennas has totransmit CSI-RS on 128 REs in one RB. Such a CSI-RS transmission schemeconsumes excessive radio resources and thus causes shortage of resourcefor data transmission.

Exemplary embodiments of the present invention are described withreference to the accompanying drawings. Detailed description ofwell-known functions and structures incorporated herein may be omittedto avoid obscuring the subject matter of the present invention. Thisaims to omit unnecessary description so as to make the subject matter ofthe present invention clear.

For the same reason, some of elements are exaggerated, omitted, orsimplified in the drawings and the elements may have sizes and/or shapesdifferent from those shown in drawings, in practice. The same referencenumbers are used throughout the drawings to refer to the same or likeparts.

Advantages and features of the present invention and methods ofaccomplishing the same may be understood more readily by reference tothe following description of exemplary embodiments and the accompanyingdrawings. Exemplary embodiments of the present invention may, however,be embodied in many different forms and should not be construed as beinglimited to the exemplary embodiments set forth herein. Rather, theseexemplary embodiments are provided so that this disclosure will bethorough and complete and will fully convey the concept of the inventionto those skilled in the art, and exemplary embodiments of the presentinvention will only be defined by the appended claims. Like referencenumerals refer to like elements throughout the specification.

It will be understood that each block of the flowchart illustrationsand/or block diagrams, and combinations of blocks in the flowchartillustrations and/or block diagrams, can be implemented by computerprogram instructions. These computer program instructions may beprovided to a processor of a general purpose computer, special purposecomputer, or other programmable data processing apparatus to produce amachine, such that the instructions, which execute via the processor ofthe computer or other programmable data processing apparatus, createmeans for implementing the functions/acts specified in the flowchartand/or block diagram block or blocks. These computer programinstructions may also be stored in a computer-readable memory that candirect a computer or other programmable data processing apparatus tofunction in a particular manner, such that the instructions stored inthe computer-readable memory produce an article of manufacture includinginstruction means which implement the function/act specified in theflowchart and/or block diagram block or blocks. The computer programinstructions may also be loaded onto a computer or other programmabledata processing apparatus to cause a series of operational steps to beperformed on the computer or other programmable apparatus to produce acomputer implemented process such that the instructions which execute onthe computer or other programmable apparatus provide steps forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

Furthermore, the respective block diagrams may illustrate parts ofmodules, segments or codes including at least one or more executableinstructions for performing specific logic function(s). Moreover, itshould be noted that the functions of the blocks may be performed indifferent order in several modifications. For example, two successiveblocks may be performed substantially at the same time, or may beperformed in reverse order according to their functions.

The term “module” according to exemplary embodiments of the presentinvention, means, but is not limited to, a software or hardwarecomponent, such as a Field Programmable Gate Array (FPGA) or anApplication Specific Integrated Circuit (ASIC), which performs certaintasks. A module may advantageously be configured to reside on theaddressable storage medium and configured to be executed on one or moreprocessors. Thus, a module may include, by way of example, components,such as software components, object-oriented software components, classcomponents and task components, processes, functions, attributes,procedures, subroutines, segments of program code, drivers, firmware,microcode, circuitry, data, databases, data structures, tables, arrays,and variables. The functionality provided for in the components andmodules may be combined into fewer components and modules or furtherseparated into additional components and modules. In addition, thecomponents and modules may be implemented such that they execute one ormore Central Processing Units (CPUs) in a device or a secure multimediacard.

It can be considered for the eNB having a plurality of transmit antennasfor FD-MIMO to transmit CSI-RSs on N dimensions such that the UEperforms channel measurements for the plurality of transmit antennaswithout excessive resource allocation for CSI-RS transmission. In theexemplary case of FIG. 1 where the transmit antennas 110 of the eNB arearranged 2-dimensionally, the CSI-RSs may be transmitted as separatedinto 2 dimensions. In this case, one CSI-RS is used as a horizontalCSI-RS for acquiring the horizontal direction channel information whilethe other CSI-RS is used as a vertical CSI-RS for acquiring verticaldirection channel information.

FIG. 3 is a diagram illustrating a mechanism of a CSI-RS transmission inan FD-MIMO system according to an exemplary embodiment of the presentinvention.

Referring to FIG. 3, the eNB operating in FD-MIMO mode has a total of 32antennas 300. Although the number of antennas may vary depending on theexemplary embodiment, FIG. 3 is directed to the exemplary case where thenumber of antennas is 32. In FIG. 3, the 32 antennas 300 are indicatedby A0, . . . , A3, B0, . . . , B3, C0, . . . , C3, D0, . . . , D3, E0, .. . , E3, F0, . . . , F3, G0, . . . , G3, and H0, . . . , H3. TwoCSI-RSs are transmitted through the 32 antennas. The antenna portscorresponding to a Horizontal-CSI-RS (H-CSI-RS) for use in measuringhorizontal channel state consist of the following 8 antenna ports.

1. H-CSI-RS port 0: group of antennas A0, A1, A2, and A3.

2. H-CSI-RS port 1: group of antennas B0, B1, B2, and B3.

3. H-CSI-RS port 2: group of antennas C0, C1, C2, and C3.

4. H-CSI-RS port 3: group of antennas D0, D1, D2, and D3.

5. H-CSI-RS port 4: group of antennas E0, E1, E2, and E3.

6. H-CSI-RS port 5: group of antennas F0, F1, F2, and F3.

7. H-CSI-RS port 6: group of antennas G0, G1, G2, and G3.

8. H-CSI-RS port 7: group of antennas H0, H1, H2, and H3.

The expression of grouping a plurality of antennas into one CSI-RS portis a concept including antenna virtualization. Typically, the antennavirtualization is through a linear combination of the plurality ofantennas. The antenna ports corresponding to a Vertical-CSI-RS(V-CSI-RS) for use in measuring vertical channel state consist of thefollowing 4 antenna ports.

1. V-CSI-RS port 0: group of antennas A0, B0, C0, D0, E0, F0, G0, andH0.

2. V-CSI-RS port 1: group of antennas A1, B1, C1, D1, E1, F1, G1, andH1.

3. V-CSI-RS port 2: group of antennas A2, B2, C2, D2, E2, F2, G2, andH2.

4. V-CSI-RS port 3: group of antennas A3, B3, C3, D3, E3, F3, G3, andH3.

It is assumed that the plurality of antennas are arranged in 2dimensions as described above. The antennas are arranged orthogonallyforming M rows in the vertical direction and N columns in the horizontaldirection. In this case, the UE is capable of measuring FD-MIMO channelsusing N H-CSI-RS ports and M V-CSI-RS ports. As aforementioned, if twoCSI-RSs are used, the channel state information can be acquired usingM+N CSI-RS ports for M×N transmit antennas. Since the channelinformation on the large number of transmit antennas is acquired usingrelatively small number of CSI-RS ports, it is advantageous in reducingthe CSI-RS overhead. Although the description is directed to the casewhere the channel information on the FD-MIMO transmit antennas using twoCSI-RSs, this approach can be applied to the cases of using two or moreCSI-RSs.

In FIG. 3, the RSs of the 32 transmit antennas are mapped to 8 H-CSI-RSports and 4 V-CSI-RS ports, and the UE measures the radio channels usingthe CSI-RSs of the FD-MIMO system. The H-CSI-RS can be used forestimating the horizontal angle between the UE and the eNB transmitantennas 310, while the V-CSI-RS can be used for estimating the verticalangle between the UE and the eNB transmit antennas 320.

Grouping multiple antenna elements into a single CSI-RS port means thatthe grouped antenna elements contribute to the transmission of theCSI-RS port. Multiple antenna elements contributing to the transmissionof a single CSI-RS antenna port is beneficial compared to having onlyone of the multiple antenna elements transmit for the rest of theantenna elements in the following aspects:

More transmission energy can be accumulated compared to having only oneantenna element transmit for the rest of the antenna elements,

Single CSI-RS antenna port can more effectively represent the groupedantenna elements since the entire antenna elements are contributing tothe CSI-RS antenna port compared to only one antenna elementtransmitting for the rest of the antenna elements. It is advantageousfor the UE to measure the averaged channel state and reports the channelstate information acquired based on the averaged channel state in viewof system performance.

As shown in FIG. 3, depending on the total number of antenna elements,different number of antenna elements can contributed to either theH-CSI-RS port or V-CSI-RS port. In the exemplary embodiment of FIG. 3, 4antenna elements contribute to a single H-CSI-RS port while 8 antennaelements contribute to single V-CSI-RS port. Since different number ofantennas contribute to an H-CSI-RS port and a V-CSI-RS port, differenttransmission powers are allocated for the H-CSI-RS port and V-CSI-RSport. Typically, the more the antenna contribute, the higher thetransmission power is.

In the case of estimating the downlink channel state using H-CSI-RS andV-CSI-RS, it is necessary for the use to know the power differencebetween H-CSI-RS port and the V-CSI-RS port. This is because the UE candetermine the channel state of horizontal and vertical direction channelstate accurately based on the accurate transmission power ratioinformation. In the exemplary embodiment of FIG. 3, the transmissionpower for the H-CSI-RS port is half of V-CSI-RS port, i.e.,vertical-to-horizontal transmission power ratio of 2:1. Without theknowledge of the transmission power ratio of 2:1 between V-CSI-RS portand H-CSI-RS port, it may be shown the horizontal direction channelstate is better than the vertical direction channel state, although itis not true. In order to prevent the UE for experiencing this channeldistortion effect, there is a need of a method for the UE to determinethe transmission power ratio between the H-CSI-RS port and V-CSI-RSport. An exemplary embodiment of the present invention proposes twomethods for determining the transmission power ratio between H-CSI-RSport and V-CSI-RS port.

Exemplary Method 1 for Determining a Transmission Power Ratio BetweenH-CSI-RS and V-CSI-RS Ports

In exemplary method 1, a UE receives the transmission power ratio of aV-CSI-RS port and H-CSI-RS port through explicit signaling anddetermines the numbers of ports for V-CSI-RS and H-CSI-RS based on thesignaling. If the number of ports for V-CSI-RS is n_(v) and H-CSI-RS isn_(h), the UE implicitly assumes that the ratio between V-CSI-RS portpower and H-CSI-RS port power, P_(h):P_(v), is n_(h):n_(v). Here, P_(v)denotes the transmission power of the V-CSI-RS port, and P_(h) denotesthe transmission power of the H-CSI-RS port. In the exemplary case ofFIG. 3, since there are 8 H-CSI-RS ports and 4 V-CSI-RS ports, the UEassumes that the ratio of V-CSI-RS port power and H-CSI-RS port powerare 2:1. For example, the UE determines at least one of a channel stateand feedback information under the assumption that the V-CSI-RS porttransmission power is twice of the H-CSI-RS transmission power.

In exemplary method 1, the transmission power ratio between V-CSI-RSport and H-CSI-RS port is determined as follows:

Step 1: The eNB notifies the UE of the configuration on the V-CSI-RS andH-CSI-RS. In this process, the eNB notifies the UE of the number ofV-CSI-RS ports (n_(v)) and H-CSI-RS ports (n_(h)) through higher layersignaling.

-   -   In step 1, the eNB may further notify the UE that the V-CSI-RS        and H-CSI-RS are of a single eNB. Such indication may be        required if a UE is required to perform measurement for multiple        eNBs. An explicit indication can be conveyed to the UE for this        notification or the UE may derive this notification based on        other parameters. For example, if the virtual or physical cell        ID value for the determination of the CSI-RS scrambling sequence        is the same, the UE may assume that two CSI-RSs are the V-CSI-RS        and H-CSI-RS of the same eNB. For example, if the virtual cell        ID used by the eNB in determining the initial state for use in        generating scrambling sequence of V-CSI-RS and H-CSI-RS is        identical, the UE determines that the V-CSI-RS and H-CSI-RS are        transmitted by the same eNB. The signal transmitted by the same        eNB may be used for determining the numbers of V-CSI-RS and        H-CSI-RS ports.

Step 2: Based on the signaled value of n_(h) and n_(v), the UEimplicitly determines the ratio between V-CSI-RS port power and H-CSI-RSport power, P_(h):P_(v), is n_(h):n_(v).

In exemplary method 1, the ratio between V-CSI-RS port power andH-CSI-RS port power can be used in conjunction with the value P_(c)defined in the LTE/LTE-A Release 10 specification. In LTE/LTE-A, the UEreceives P_(c) which notifies of ratio between PDSCH transmission powerand CSI-RS transmission power. By applying P_(c) to the measurement ofCSI-RS, the UE determines the expected SNIR (signal to noise andinterference ratio) on the PDSCH. P_(c) can be applied in using theratio between the V-CSI-RS port transmission power and V-CSI-RS porttransmission power in exemplary method 1. In this case the UE measuresSINR with V-CSI-RS and H-CSI-RS using the transmission power ratiobetween the V-CSI-RS port and H-CSI-RS port. Afterward, the UEcalculates SINR of PDSCH using SINR.

Exemplary Method 2 for Determining a Transmission Power Ratio BetweenH-CSI-RS and V-CSI-RS Ports

In exemplary method 2, a UE determines the transmission power ratio of aV-CSI-RS port and H-CSI-RS port based an explicit higher layer signalingfrom the eNB. There are two different methods for signaling thisinformation.

Exemplary method 2-1: The eNB notifies the UE of

$\frac{P_{h}}{P_{v}}.$

Exemplary method 2-2: The eNB notifies the UE of

${P_{c,h} = {{\frac{P_{h}}{P_{d}}\mspace{14mu} {and}\mspace{14mu} P_{c,v}} = \frac{P_{v}}{P_{d}}}}\mspace{14mu}$

respectively. P_(d) denotes a PDSCH transmission power. The UE receivesP_(c,h) and P_(c,v) and calculates the ratio between H-CSI-RS porttransmission power and V-CSI-RS port transmission power based on P_(c,h)and P_(c,v).

In exemplary method 2, the transmission power ratio between the H-CSI-RSport and V-CSI-RS port are determined as follows:

Step 1: The eNB sends the UE V-CSI-RS and H-CSI-RS configuration throughhigher layer signaling. In the process, the eNB notifies the UE of

${\frac{P_{h}}{P_{d}}\mspace{14mu} {and}\mspace{14mu} {\frac{P_{v}}{P_{d}}.}}\mspace{14mu}$

Step 2: The UE determines the ratio of V-CSI-RS port power and H-CSI-RSport power based on the signaled value of

${\frac{P_{h}}{P_{d}}\mspace{14mu} {and}\mspace{14mu} {\frac{P_{v}}{P_{d}}.}}\mspace{14mu}$

In order to report the channel state information for FD-MIMO, there is aneed of an efficient channel state information feedback method as wellas the method for determining a transmission power ratio betweenH-CSI-RS port and V-CSI-RS port. For example, the UE has to performchannel estimation for the horizontal and vertical dimension channelestimations based on the V-CSI-RS and H-CSI-RS and report the channelestimation results to the eNB effectively and efficiently.

FIG. 4 is a diagram illustrating a procedure of estimating channelsbased on H-CSI-RS and V-CSI-RS and generating channel state informationaccording to an exemplary embodiment of the present invention.

Referring to FIG. 4, the eNB transmits H-CSI-RS and V-CSI-RSalternatively. Reference number 400 denotes H-CSI-RS and referencenumber 410 denotes V-CSI-RS. The eNB transmitting the H-CSI-RS andV-CSI-RS for FD-MIMO transmission is capable of transmitting data toboth the FD-MIMO-capable UE and non-FD-MIMO-capable UE. In order toaccomplish this, the eNB has to discriminate between the channel stateinformation transmitted by the FD-MIMO-capable UE and thenon-FD-MIMO-capable UE. In an exemplary embodiment of the presentinvention, the H-CSI-RS 400 and V-CSI-RS 410 may be transmitted at thesame timing.

In FIG. 4, the eNB configures such that the non-FD-MIMO-capable UEmeasures only one of H-CSI-RS and V-CSI-RS. For example, it is possibleto instruct the non-FD-MIMO-capable UE to measure H-CSI-RS and reportCSI ad denoted by reference number 420. In the case of operating theFD-MIMO with the channel state information acquired based on only one ofthe H-CSI-RS and V-CSI-RS, it is difficult to expect the best FD-MIMOperformance. Nevertheless, it allows for data transmission. Meanwhile,the FD-MIMO-capable UE measures both the H-CSI-RS and V-CSI-RS and thus,it is possible to acquire improved system performance.

There are three methods for the UE to generate channel stateinformation. The first method is to measure the H-CSI-RS and V-CSI-RSrespectively and acquire the respective horizontal and verticaldimension channel state information from the H-CSI-RS and V-CSI-RS asdenoted by reference number 430. The second method is to measure theH-CSI-RS and V-CSI-RS respectively and acquire a single set of channelstate information as denoted by reference number 440. The third methodis to measure the H-CSI-RS and V-CSI-RS respectively, acquire thechannel state information based on one of the H-CSI-RS and V-CSI-RS, andthe channel state information based on the other as denoted by referencenumber 450. In the case of the method denoted by reference number 450,the first channel state information is acquired based on the V-CSI-RSand the second channel state information is acquired using the H-CSI-RSin addition.

The three channel state information generation methods can be summarizedas follows:

CSI Generation Exemplary Method 1

In this method, the channel state information of H-CSI-RS and V-CSI-RSare determined individually. For example, the channel state informationbased on H-CSI-RS does not impact the result of the channel stateinformation based on V-CSI-RS and vice versa. This method generates twosets of channel state information that are individually optimized in thehorizontal or vertical direction but not both. The UE measures H-CSI-RSand determines the channel state information based on H-CSI-RS withoutconsideration for V-CSI-RS. Additionally, the UE measures V-CSI-RS anddetermines the channel state information based on V-CSI-RS withoutconsideration for H-CSI-RS. However, either of the two sets of channelstate information may not be optimized for the 2 dimensional channelconsidering the horizontal and vertical direction channels integrally.

This method is advantageous in that the erroneous reception of one ofthe set of channel state information does not impact the other set ofchannel state information. For example, although it fails to receive thechannel status information of the horizontal direction, the eNB alwayshas the option of falling back to the channel status information in thevertical direction.

CSI Generation Exemplary Method 2

In this method, the channel state information based on H-CSI-RS andV-CSI-RS are determined together as single set of channel stateinformation. This method generates a single set of channel stateinformation that is optimized for both the horizontal and verticaldirection. The UE measures the H-CSI-RS and V-CSI-RS and determines thechannel state information in consideration of both dimensions. Thedrawback of this method is high channel state information determinationcomplexity. This is because the optimization has to be performed in thedimension of n_(h)+n_(v) in order to take the vertical and horizontaldirection channels into consideration in the case that there are n_(h)H-CSI-RS and n_(v) V-CSI-RS ports. Typically, the complexity ofoptimization performed to n_(h) and n_(v) separately is lower than thatperformed to n_(h)+n_(v).

CSI Generation Exemplary Method 3

In this method, the channel state information based on one of H-CSI-RSand V-CSI-RS is determined first and the channel state information basedon the remaining CSI-RS is determined. For example, the H-CSI-RS wouldbe measured first and the corresponding channel state information isdetermined. In addition, the UE generate additional channel stateinformation in consideration of both the horizontal and verticaldirections using the channel state information acquired based on theH-CSI-RS and the V-CSI-RS.

In addition to using one of the three methods, any combination of threemethods can be used. For example, exemplary method 1 can be used toprovide the individual channel state information on the horizontal andvertical direction. In addition to the individual channel stateinformation on both directions, an additional set of information thatare determined by measuring both H-CSI-RS and V-CSI-RS can be defined.The purpose of the additional set of information is to provide accuratechannel status information for the case where both antenna ports areactivated on both dimensions for a UE.

An exemplary embodiment of the present invention proposes a channelstate information feedback method under the assumption that the H-CSI-RSand V-CSI-RS are transmitted for channel measurement in the horizontaland vertical directions. Although the description is directed to themethods assuming a certain timing, the general concept is applicable todiverse timing cases.

The following abbreviations are used throughout the specification.

RI_(H): a Rank Indicator (RI) generated based on H-CSI-RS for feedbackto eNB.

RI_(V): an RI generated based on V-CSI-RS for feedback to eNB.

RI_(HV): an RI generated based on H-CSI-RS and V-CSI-RS for feedback toeNB.

PMI_(H): a Precoding Matrix Indicator (PMI) generated based on H-CSI-RSfor feedback to eNB.

PMI_(V): a PMI generated based on V-CSI-RS for feedback to eNB.

CQI_(H): a UE-recommended data rate generated under the assumption thatonly the horizontal direction precoding matrix is applied.

CQI_(V): a UE-recommended data rate generated under the assumption thatonly the vertical precoding matrix is applied.

CQI_(HV): a UE-recommended data rage generated under the assumption thatboth the horizontal and vertical precoding matrices are applied.

In the following, the description is directed to the case of using thehorizontal direction channel state information and the verticaldirection channel state information. In the case that an eNB operateswith two or more CSI-RSs, however, other types of channel stateinformation can be further applied in addition to the horizontal andvertical direction channel state information. In an exemplary case wherethe CSI-RS mapped to an antenna port from the first view point (firstCSI-RS) and the CSI-RS mapped to an antenna port from the second viewpoint (second CSI-RS) are used, the UE is capable of acquiring thechannel state information (first and second channel state information)based on the two respective CSI-RSs and the channel state information(third channel state information) based on both the CSI-RSs. Theconfiguration described in the following description are application tovarious exemplary embodiments in similar manner. In the following, thedescription is directed to the exemplary cases of using V-CSI-RS andH-CSI-RS for the convenience sake.

In the following description, the channel state informationcorresponding to the vertical direction CSI-RS is referred to asvertical direction channel state information. The vertical directionchannel state information includes at least one of RI, PMI, and CQIacquired based on the vertical direction CSI-RS.

In the following description, the channel state informationcorresponding to the horizontal direction CSI-RS is referred to ashorizontal direction channel state information. The horizontal channelstate information includes at least one of an RI, a PMI, and a CQIacquired based on the horizontal direction CSI-RS.

In the case that the eNB sends the UE two or more CSI-RSs, the UE iscapable of transmitting the channel state information corresponding tothe respective CSI-RSs. Each of the channel state information includesat least one of an RI, a PMI, and a CQI. However, the UE may acquire thechannel state information based on the two or more CSI-RSs in anexemplary embodiment of the present invention. In this case, how thechannel state information is acquired is described explicitly.

In an exemplary embodiment of the present invention, the UE may notifyof the vertical direction precoding and horizontal direction precodingseparately to determine the precoding for the case of using both thevertical and horizontal direction precodings. It is necessary to definehow to determine CQI in the case that a plurality of precodings areapplied. In the case that two precodings are applied to downlinksimultaneously, it can be considered as Kronecker product of the twoprecodings. The Kronecker product is defined with two matrices asfollows:

$\begin{matrix}{{{A \otimes B} = \begin{bmatrix}{a_{11}B} & \ldots & {a_{1\; n}B} \\\vdots & \ddots & \vdots \\{a_{m\; 1}B} & \ldots & {a_{mn}B}\end{bmatrix}},} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

In Equation (1), A and B denote matrices, and a₁₁ to a_(mn) denoteelements of matrix A. a_(ij) denotes the element at i^(th) row andj^(th) column.

CSI Feedback Exemplary Method 1

FIG. 5 is a timing diagram illustrating a channel state informationfeedback method 1 according to an exemplary embodiment of the presentinvention.

Referring to FIG. 5, in the channel state information feedback method,the UE generates two sets of channel state information to the eNB. Thetwo sets of channel state information are generated based on theH-CSI-RS and V-CSI-RS transmitted by the eNB. As shown in FIG. 5, afirst set of channel state information consists of RI_(H) 500, CQI_(H)510, and CQI_(V) 520. For example, the UE generates horizontal directionRI, PMI, and CQI based on the H-CSI-RS and vertical direction RI_(V)505, PMI_(H) 515, and PMI_(V) 525 based on the V-CSI-RS. A second set ofchannel state information consists of RI_(H) 530, CQI_(H) 540, andCQI_(V) 550. For example, the UE generates horizontal direction RI, PMI,and CQI based on the H-CSI-RS and vertical direction RI_(V) 535, PMI_(H)545, and PMI_(V) 555 based on the V-CSI-RS.

In LTE, CQI may indicate UE's maximum receivable data rate or spectralefficiency. In radio communication system including LTE, the UEgenerates one CQI to the eNB for RI set 1 and two or more CQIs for RIset to 2 or higher. Assuming that CQI is transmitted, if RI is set to 2or higher, this means that the corresponding CQI has two CQI values. InFIG. 5, if RI_(H) 500 is set to 2, the CQI_(H) 510 consists of two CQIvalues.

In FIG. 5, the UE measures H-CSI-RS and V-CSI-RS to generate and sendRI_(H) 500 and RI_(V) 505 to the eNB. If the preferred precoding isdetermined by measuring H-CSI-RS and if the determined precoding isapplied, the maximum data rate or spectral efficiency is determined. TheUE notifies the eNB of the determined preferred precoding and maximumdata rate or spectral efficiency using PMI 510 and CQI 515. The UE alsosends the eNB the PMI 520 and CQI 525 acquired based on V-CSI-RS.

If the two sets of channel state information are received from the UE,the eNB determines the precoding of PDSCH to be transmitted to the UEbased on the precoding-related channel state information. For example,the eNB determines the precoding matrix to be applied to PDSCH to betransmitted in downlink based on PMI_(H) and PMI_(V) transmitted by theUE.

The CQI transmitted by the UE is generated under the assumption of acertain precoding to be applied at the eNB. In LTE, the UE sends PMI tothe eNB to notify of the recommended precoding, and CQI indicatesUE-recommended maximum data rate or spectral efficiency in the case thatthe corresponding PMI is applied at the eNB. In the case that twoCSI-RSs exist and the precoding is applied in horizontal and verticaldirections simultaneously, it is necessary to define new CQI. CQI_(H)can be defined as one of the followings.

CQI_(H) definition 1: The UE generates CQI_(H) regardless of thepresence of vertical direction antennas. For example, the UE generatesCQI_(H) only based on H-CSI-RS but not V-CSI-RS. More specifically, theUE generates its best PMI_(H) based on the H-CSI-RS and notifies the eNBof the maximum data rate or spectral efficiency, when the correspondingPMI is applied, by transmitting CQI_(H).

CQI_(H) definition 2: The UE generates CQI_(H) under the assumption offixed precoding in the vertical direction. For example, the UE generatesCQI_(H) under the assumption that a fixed rank 1 precoding is applied tothe vertical direction antennas. This CQI_(H) generation is identicalwith determining CQI under the assumption that the precoding ofG_(HV)=G_(H) {circle around (×)}G_(V) to which both the horizontal andvertical direction precodings are applied. In G_(HV)=G_(H) {circlearound (×)}G_(V), G_(H) denotes the horizontal direction precoding fedback from the UE to the eNB using PMI_(H), and G_(V) denotes the fixedvertical direction rank 1 precoding. In addition, {circle around (×)}denotes Kronecker product. The fixed vertical direction precoding can bethe precoding specified in the standard or notified to the UE by eNBthrough higher layer signaling.

CQI_(H) definition 3: The UE generates CQI_(H) under the assumption thatthe precoding negotiated with the eNB in the vertical direction isapplied. For example, the eNB and the UE assume that vertical directionprecoding is one of a plurality of precoding candidates and determinedaccording to the method negotiated therebetween. For example, assumingvertical direction precoding candidates of {G_(V1), G_(V2), G_(V3),G_(V4)}, when generating CQI_(H), the UE assumes vertical directionprecoding of G_(V1) at time 1, assumes vertical direction precoding ofG_(V2) at time 2, assumes vertical direction precoding of G_(V3) at time3, and assumes vertical direction precoding of G_(V4) at time 4according to the pre-negotiated method. The vertical direction precodingcan be changed in time domain or frequency domain. In the case that thevertical direction precoding is changed in the frequency domain, the UEdivides the system bandwidth assumes that a specific vertical directionprecoding is applied to a specific frequency band and other verticaldirection precodings are applied to other frequency bands. For example,CQI is generated under the assumption that the precoding represented byG_(HV)=G_(H) {circle around (×)}G_(V(i)) is applied. In G_(HV)=G_(H){circle around (×)}G_(V(i)) is one of the precodings assumable in thevertical direction. The change G_(V(i)) can be determined through cyclicrotation in the given set of G_(V(i)).

The CQI_(H) definition methods can be applied to CQI_(V).

The drawback of exemplary channel state information feedback method 1 isthat, although the UE recommends its preferred vertical and horizontaldirection precodings to the eNB, not both the two precodings are appliedto the CQI generation. For example, the UE does not report the CQIacquired when the best vertical and horizontal direction precodings areapplied and thus, although the eNB transmits PDSCH with the bestvertical and horizontal direction precodings, it is difficult the datarate supported by the UE. Although the eNB may determine the data ratewhen the best vertical and horizontal direction precodings are appliedwith the compensation of CQI_(H) or CQI_(V) in itself, the determineddata rate may differ from the data rate supportable by the UE. Such amismatch occurs because the eNB does not know the receiver structure ofthe UE.

CSI Feedback Exemplary Method 2

FIG. 6 is a timing diagram illustrating a channel state informationfeedback method 2 according to an exemplary embodiment of the presentinvention.

Referring to FIG. 6, in the channel state information feedback method,the UE generates two sets of channel state information to the eNB as inthe feedback method of FIG. 5. In the exemplary channel stateinformation method 2, however, CQI_(HV) 630 (665) is further transmittedto the eNB. The CQI_(HV) 630 (665) is the information on the maximumdata rate or spectral efficiency supportable by the UE when the bestvertical and horizontal direction precodings are applied. The UEgenerates CQI_(HV) 630 under the assumption that the precodingdetermined according to the RI_(H) 600 and PMI_(H) 615 based on H-CSI-RSand RI_(V) 605 and PMI_(V) 625 based on V-CSI-RS is applied. A first setof channel state information consists of RI_(H) 600, CQI_(H) 610, andCQI_(V) 620. A second set of channel state information consists ofRI_(H) 635, CQI_(H) 645, and CQI_(V) 655. For example, the UE generateshorizontal direction RI, PMI, and CQI based on the H-CSI-RS and verticaldirection RI_(V) 640, PMI_(H) 650, and PMI_(V) 660 based on theV-CSI-RS. For example, CQI_(HV) 630 (665) is the information on themaximum data rate or spectral efficiency when the UE-recommendedvertical and horizontal direction precodings are applied simultaneously.This method is advantageous in improvement of system performance sincethe UE sends the eNB the information on the case where the best verticaland horizontal precodings are applied simultaneously.

In the case that the UE transmits CQI_(HV) 630 (665) as shown in theexemplary embodiment of FIG. 6, one issue to be considered is how todetermine RI. In LTE, the UE transmits one RI for use in one CSI processto notify the eNB that how many spatial layers the UE is supportable.One problem associated with RI in FIG. 6 is that two RIs (RI_(H) andRI_(V)) exist. This is because it is necessary to define the rank inorder to report accurate CQI_(HV) 630 (665) even when the vertical andhorizontal direction precodings are applied and there are two RIsredundantly.

RI_(HV) definition exemplary method 1: RI_(HV)=RI_(H)×RI_(V).

RI_(HV) definition exemplary method 2: RI_(HV)=max(RI_(H), RI_(V)).

RI_(HV) definition exemplary method 3: RI_(HV)=min(RI_(H), RI_(V)).

RI_(HV) definition exemplary method 4: RI_(HV)=RI_(H) or RI_(HV)=RI_(V).

In RI_(HV) definition exemplary method 1, RI_(HV) is defined as theproduct of RI_(H) and RI_(V). The precoding for this case is obtained bythe Kronecker product of the precoding matrices indicated by PMI_(H) andPMI_(V). Therefore, the precoding for the case of exemplary method 1 isdefined as G_(HV)=G_(V){circle around (×)}G_(H) or G_(HV)=G_(H){circlearound (×)}G_(V) where G_(H) and G_(v) are the precoding matricesindicated by PMI_(H) and PMI_(V), respectively.

In RI_(HV) definition of exemplary method 2, RI_(HV) is defined as themaximum of RI_(H) and RI_(V). The precoding for this case is againobtained by the Kronecker product of the precoding matrices. However,since RI_(HV) is defined as the maximum of RI_(H) and RI_(V), not allthe columns of the precoding matrix with the lower rank can be used forthe definition of G_(HV). Therefore,

If RI_(H)≧RI_(V), the precoding for the RI_(HV) definition of exemplarymethod 2 is defined as G_(HV)=G_(V)′{circle around (×)}G_(H) orG_(H){circle around (×)}G_(V)′ where G_(V)′ is the precoding matrix onthe vertical dimension obtained by selecting 1 column out of the RI_(V)columns of G_(V). The column to be selected is fixed or predefined.

If RI_(H)<RI_(V), the precoding for the RI_(HV) definition of exemplarymethod 2 is defined as G_(HV)=G_(V){circle around (×)}G_(H)′ orG_(H)′{circle around (×)}G_(V) where G_(H)′ is the precoding matrix onthe horizontal dimension obtained by selecting 1 column out of theRI_(H) columns of G_(H). The column to be selected is fixed orpredefined.

In RI_(HV) definition of exemplary method 3, RI_(HV) is defined as theminimum of RI_(H) and RI_(V). The precoding for this case is againobtained by the Kronecker product of the precoding matrices. However,since RI_(HV) is defined as the minimum of RI_(H) and RI_(V), not allthe columns of the precoding matrix with the higher rank can be used forthe definition of G_(HV). Therefore,

If RI_(H)<RI_(V), the precoding for RI_(HV) definition of exemplarymethod 3 is defined as G_(HV)=G_(V)′{circle around (×)}G_(H) orG_(H){circle around (×)}G_(V)′ where G_(V)′ is the precoding matrix onthe vertical dimension obtained by selecting 1 column out of the RI_(V)columns of G_(V). The column to be selected is fixed or predefined.

If RI_(H)≧RI_(V), the precoding for RI_(HV) definition of exemplarymethod 3 is defined as G_(HV)=G_(V){circle around (×)}G_(H)′ orG_(H)′{circle around (×)}G_(V) where G_(H)′ is the precoding matrix onthe horizontal dimension obtained by selecting 1 column out of theRI_(H) columns of G_(H). The column to be selected is fixed orpredefined.

In RI_(HV) definition of exemplary method 4, RI_(HV) is defined aseither RI_(H) or RI_(V). This can be achieved by fixing either RI_(H) orRI_(V) to be 1. When either RI_(H) or RI_(V) is fixed to the value of 1,its value can be omitted in the channel status information feedbacksince this information is already known by the eNB. For example, in FIG.6, if rank in the horizontal direction is fixed as 1, it is notnecessary to send RI_(H) values. Typically, the eNB can inform the UEusing higher layer signaling which of the two dimensions should berestricted to a rank of 1. When the rank of one of the dimensions isrestricted to one, a simple Kronecker product of this precoding matrixand the other precoding matrix which does not have this restriction willgenerate G_(HV).

In FIG. 6, CQI_(HV) is related to either CQI_(H) or CQI_(V). AlthoughEither CQI_(H) or CQI_(V) has the CQI value to which the preferredprecoding of either direction is applied, CQI_(HV) has the CQI value towhich the preferred precodings of both the vertical and horizontaldirections are applied. Typically, CQI_(HV) has a value greater thanCQI_(H) or CQI_(V). By taking notice of this characteristic, there is aroom for reducing the information amount of CQI_(HV). For example, it ispossible to notify the eNB of the CQI_(HV) as a relative value toCQI_(H) or CQI_(V) other than absolute value of CQI_(HV).

CSI Feedback Exemplary Method 3

FIG. 7 is a timing diagram illustrating a channel state informationfeedback method 3 according to an exemplary embodiment of the presentinvention.

Referring to FIG. 7, the exemplary channel state information feedbackmethod 3 is characterized in that only one RI is transmitted. In FIG. 7,RI 700 (or 740) indicates the number of layers that can be supported bythe UE in the vertical or horizontal direction. In the exemplary channelstate information feedback method 3, the eNB notifies the UE of thedirection in which the rank is fixed to 1, and the UE determines therank in the direction in which the rank is not fixed and sends thedetermined RI to the eNB. According to an exemplary embodiment, aspecific CQI value may be transmitted at the same timing as the PMI.More specifically, CQI_(HV) 730 (770) may be transmitted at the sametiming as one of PMI_(H) 710 (750) and PMI_(V) 720 (760).

CSI Feedback Exemplary Method 4

FIG. 8 is a timing diagram illustrating a channel state informationfeedback method 4 according to an exemplary embodiment of the presentinvention.

Referring to FIG. 8, in the exemplary channel state information feedbackmethod 4, the UE sends eNB the channel status information generated forthe case where the preferred precodings of the vertical and horizontaldirections are applied and the channel status information generated forthe case where the preferred precoding of either the vertical directionor the horizontal direction.

The exemplary channel state information feedback method 4 aims toprovide PMI and CQI on one of vertical and horizontal directions and, ontop of this, the channel state information for the case where thepreferred precoding is applied for the other direction. In FIG. 8, theUE sends the eNB the channel status information for the horizontaldirection. At this time, the horizontal direction channel stateinformation generated by the UE includes CQI_(H) 805 (830) and PMI_(H)810 (835). In addition, the UE transmits the vertical directionpreferred precoding information 815 (840) and the CQI_(HV) 820 (845) forthe case where the vertical and horizontal direction preferredprecodings are applied simultaneously. Similar to the exemplary channelstate information feedback method 1, in order to avoid the complexitydue to the coexistence of RI_(H) and RI_(V), only one direction RI istransmitted. In the exemplary embodiment of FIG. 8, RI 800 (825) is usedfor notifying the eNB of the vertical direction rank while thehorizontal direction rank is fixed to 1. According to an exemplaryembodiment, the rank value for the direction with the least channelvariation may be fixed to 1.

FIG. 9 is a block diagram illustrating a configuration of an eNBaccording to an exemplary embodiment of the present invention.

Referring to FIG. 9, the eNB comprises a controller 900, an H-CSI-RSgenerator 910, a V-CSI-RS generator 920, other signal generator 930, amultiplexor 940, and an OFDMA signal generator 950. The H-CSI-RSgenerator 910 and the V-CSI-RS generator 920 generate H-CSI-RS andV-CSI-RS respectively under the control of the controller 900. TheH-CSI-RS and V-CSI-RS are multiplexed by the multiplexer 940 andtransmitted by means of the OFDMA signal generator 950. The eNB mayfurther include a channel state information receiver to receive thechannel state information generated based on the H-CSI-RS and V-CSI-RSat the UE. According to an exemplary embodiment, the OFDMA signalgenerator 950 and the receiver may be included in the eNB in the form ofa transceiver for transmitting and receiving signal to and from the UE.

FIG. 10 is a block diagram illustrating a configuration of a UEaccording to an exemplary embodiment of the present invention.

Referring to FIG. 10, the UE includes a controller 1020, an H-CSI-RSreceiver 1030, a V-CSI-RS receiver 1040, other signal receiver 1050, ademultiplexer 1010, and an OFDMA signal receiver 1000. The UE furtherincludes an RI/PMI/CQI generator 1060 for generating channel stateinformation (CSI) and a CSI transmitter 1070 for transmitting thechannel state information.

The UE receives the signal transmitted by the eNB using the OFDMA signalreceiver 1000. The received signal is demultiplexed by the demultiplexer1010. For example, the demultiplexer 1010 demultiplexes the receivedsignal into H-CSI-RS, V-CSI-RS and other signals and delivers H-CSI-RS,V-CSI-RS and other signals to the H-CSI-RS receiver 1030, V-CSI-RSreceiver 1040, and other signal receiver 1050. The H-CSI-RS receiver1030 and the V-CSI-RS receiver 1040 perform channel measurement based onthe received H-CSI-RS and V-CSI-RS respectively, the RI/PMI/CQIgenerator 1060 generates RI/PMI/CQI based on the measurement result. TheRI/PMI/CQI is transmitted to the eNB by the CSI transmitter 1070.According to an exemplary embodiment, the OFDMA receiver 1000 and thechannel state information transmitter 1070 may be included in the UE inthe form of a transceiver for transmitting and receiving signals to andfrom the eNB.

FIG. 11 is a flowchart illustrating an eNB procedure according to anexemplary embodiment of the present invention.

Referring to FIG. 11, the eNB determines whether the UE is aFD-MIMO-capable UE at step 1110. According to an exemplary embodiment ofthe present invention, the eNB is capable of determining whether the UEhas FD-MIMO capability based on signal exchange with the UE orpredefined information.

If the UE is non-FD-MIMO-capable UE, the eNB configures one of V-CSI-RSand H-CSI-RS and transmits the configured reference signal to the UE atstep 1115.

If the UE is the FD-MIMO-capable UE, the eNB configures both theV-CSI-RS and H-CSI-RS and transmits the configured reference signals tothe UE at step 1120.

The V-CSI-RS and H-CSI-RS can be configured according to at least one ofthe above described configuration methods.

The eNB transmits a CSI-RS feedback configuration to the UE at step1125. The CSI-RS feedback can be configured according to at least one ofthe above described configuration methods. According to an exemplaryembodiment of the present invention, the eNB may perform the operationsof steps 1120 and 1125 simultaneously or separately.

The eNB receives feedback information from the UE at step 1130.

The eNB communicates FD-MIMO signals with the UE at step 1135 based onthe feedback information received from the UE at step 1130.

FIG. 12 is a flowchart illustrating a UE procedure according to anexemplary embodiment of the present invention.

Referring to FIG. 12, the UE receives at least one of V-CSI-RS andH-CSI-RS from the eNB at step 1210. According to an exemplaryembodiment, if it has the FD-MIMO capability, the UE is capable ofreceiving the V-CSI-RS and H-CSI-RS, and otherwise, only one of theV-CSI-RS and H-CSI-RS. This exemplary embodiment is directed to the caseof the UE having the FD-MIMO capability. According to an exemplaryembodiment of the present invention, the V-CSI-RS and H-CSI-RS can beconfigured according to one of the above-described methods.

The UE receives the CSI-RS feedback configuration transmitted by the eNBat step 1215. The CSI feedback can be configured according to one of theabove-described methods. In an exemplary embodiment of the presentinvention, the UE may receive the information of steps 1210 and 1215simultaneously or separately.

The UE generates feedback information based on at least one of theinformation received at step 1220.

The UE transmits the feedback information to the eNB at step 1225.

The UE communicates FD-MIMO signals with the eNB at step 1230. TheFD-MIMO signals can be transmitted/received based on the feedbackinformation transmitted at step 1225.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A signal communication method of a base stationin a wireless communication system, the method comprising: identifying afirst antenna port set and a second antenna port set; transmitting, to aterminal, at least one reference signal for the terminal; transmitting,to the terminal, feedback configuration information comprising a valueof rank indicator (RI) for the first antenna port set; and receiving,from the terminal, feedback information based on the at least onereference signal and the feedback configuration information, wherein thefeedback information includes a value of RI for the second antenna portset identified based on the at least one reference signal and the valueof RI for the first antenna port set.